The distribution of transition elements in the system CaMgSi 2O 6Na 2Si 2O 5H 2O at 1,000 bars pressure

Abstract

This study describes the results of experiments relating to the distribution of trace amounts of first row transition metal cations between diopside crystals and coexisting silicate liquid in a synthetic hydrothermal silicate system. The phase relations were determined for the system CaMgSi 2O 6Na 2Si 2O 5H 2O at 1,000 bars pressure. This system is characterized by a large field of diopside + liquid + vapour which extends to very sodium disilicate-rich portions of the system. An unknown incongruently melting phase is also present. The partition of the various transition elements between diopside crystals and coexisting silicate liquid was determined using an electron microprobe analyser. It was found that three elements, V, Cr and Ni became strongly enriched in the diopside crystals with decreasing temperature. Titanium exhibited a small tendency towards enrichment in the diopside with falling temperature. Cobalt was enriched in the diopside but was, nevertheless, rather insensitive to changes in temperature. With decreasing temperature from 1,070 to 925°C, this element displayed a slight tendency to leave the diopside, however, with temperatures decreasing from 925°C, there was a slight tendency towards increased enrichment in the diopside. All these results have been accounted for in terms of ligand field effects, bearing in mind the structural environment of the various transition metal ions in the silicate liquid as determined from absorption spectra of glasses. Thermodynamic data calculated from the experimental results further indicate the dominant importance of ligand field effects. Values obtained for ΔH° for exchange reactions involving the substitution of Mg in diopside by transition elements reflect the relative octahedral site preference energies for the transition metal cations. Values of ΔS° 1198 for the exchange reactions indicate that Cr 3+ was present essentially in one type of site in the silicate liquids (i.e., in octahedral sites) whereas Co 2+ and Ni 2+ were distributed over a number of different sites (i.e., both tetrahedral and octahedral sites). It was concluded that transition element distribution in a silicate system may be adequately predicted if details concerning its environment in the melt and the ligand field properties are known. It was noted that octahedral site preference energies should be used with caution in very high temperature portions of some silicate systems.